Abstract: The invention concerns a plastic screwable closure cap which possesses a bung type internal seal (4) for sealing a closeable container. The internal seal comprises a narrow sealing portion with an insert portion (6) immediately beneath, said insert portion serving to centre and gently introduce the internal seal. In order to attain the earliest possible venting of the closure cap when screwing off, venting recesses are provided on the insert zone (6). These prevent the insert portion from sealing the container, and enable the release of gas from the container as soon as the sealing portion comes out of engagement with the container mouth. The side surfaces of the vent recesses also serves as friction surfaces, in order to generally flatten irregularities on the container mouth before the actual sealing portion is engaged.